A method for micronization of a dispersion of particles comprising a protein having a predetermined level of biological activity. The method comprises introducing the dispersion into a vortex chamber milling apparatus under milling conditions which result in a protein powder having a particle size distribution of 5 to 100 µm and/or exhibiting a 30 to 400 fold size reduction of the protein particle dispersion from its original size, and retaining at least 80% of the predetermined level of biological activity of the protein. The milling conditions include one or more parameters selected from the following: input pressure between 1 and 7 Bars； injector pressure between 0.2 and 5 Bars； loading rate between 0.1 and 5 kg/hour； and gas flow between 30 and 100 m/hour.
